This invention relates to reduction of the traffic load from a congested cell in a mobile communications system.
In mobile communications systems mobile stations and base transceiver stations may set up connections through the channels of a so-called radio interface. A certain frequency range is always allocated for use by the system. To obtain sufficient capacity in the mobile communications system on this limited frequency band, the channels in use must be used several times. For this reason, the coverage area of the system is divided into cells formed by the radio coverage areas of individual base transceiver stations BTS, for which reason the systems are also often called cellular radio systems.
FIG. 1 shows the network elements and the relations between them in a known mobile communications system. The network shown is accordant with a GSM system, which is used as an example in the present application. The solid lines in the figure depict connections including both signalling and call connections while the dashed lines show connections including signalling only. The network includes base transceiver stations BTS which via a radio path may set up connections with the mobile stations MS of mobile station subscribers, base station controllers BSC controlling the base transceiver stations, and mobile services switching centres MSC. Hierarchically below the MSC there are several base station controllers BSC and below these there are several base transceiver stations BTS. The interface between MSC and BSC is called interface A while the interface between BSC and BTS is called interface A-bis.
Call connections which have been set up pass from base transceiver station BTS through base station controller BSC to mobile services switching centre MSC. MSC connects calls to its subordinated base station controllers, to other MSC centres or to a public switched telephone network PSTN or to an integrated services digital network ISDN. The network also includes a network management system NMS, which may be used for collecting information on the condition of the network and for supplying information and programmes to other network elements.
In the idle state, mobile stations measure the signals sent by base transceiver stations and when required they will request a connection setup from the base transceiver station which is serving best at each time. During the connection the network may move the mobile station to another cell through handover between cells whenever required and without disconnecting.
In an active call state, mobile station MS sends measuring results regularly as a report message through the serving base transceiver station BTS to base station controller BSC. The report message includes the measuring results of signal strengths of the serving base transceiver station and of no more than six adjacent base transceiver stations providing the best signal. Besides the mobile station, the base transceiver station BTS also performs measurements of the connection quality. The results of measurements performed by mobile stations and base transceiver stations are analysed in the base station controller BSC. The base station controller also maintains information on free channels in base transceiver stations of its subordinated cells. In a GSM system, the management of radio resources is almost entirely the responsibility of the base station controller BSC.
A mobile station is moved through handover from the serving cell to some adjacent cell e.g. when
The measuring results of a mobile station/base transceiver station indicate a low signal level and/or quality of the present serving cell and a better signal level can be obtained from an ambient cell,
some ambient cell allows communications at lower transmission power levels,
when a mobile station MS has moved too far from the serving base transceiver station BTS, or when
there is too much load in the serving cell.
Handover may also be done for some other reason, e.g. due to trouble in a base transceiver station. Factors affecting the choice of target cell in handover are e.g. the signal level and/or load of the target cell. To ensure stability of the mobile communications network, the measuring results and parameters used in handover are averaged over a certain time slot. In this way, handover is made less susceptible to distorted measuring results that may be caused by temporary interference or fading.
Handovers may be done
Inside a cell (intra-cell handover),
between two cells subordinated to the same base station controller (handover between base transceiver stations),
between cells subordinated to two base station controllers subordinated to the same mobile services switching centre MSC (handover between base station controllers), or
between cells subordinated to two different mobile services switching centres MSC (handover between mobile services switching centres).
Handovers are almost solely the responsibility of the base station controller BSC. The mobile services switching centre MSC participates only in such handovers between base station controllers which are due to loading of the cell.
This invention relates to such a handover due to excessive cell loading the basic principle of which is illustrated in FIG. 2. The figure shows seven cells, cells A-G, wherein the load situations are different. Cell A is loaded to the extreme limits of its capacity. Cell C is loaded a little more lightly than cell A. The loading of cells B and E is normal in view of their capacity, while cells D, F and G are lightly loaded. The load situation is proportionate to the thickness of oblique lines in the cells shown in the figure. From the viewpoint of the whole, the optimum network operation is achieved when the loads of all cells are on the same level, or when at least all cells have resources for setting up new connections. To achieve this situation, the load of cell A is discharged through handovers indicated by arrows and due to the loading of the cell into cells D, F and G which are lightly loaded.
With the aid of handover due to excessive cell loading, room for a new connection can be made in a cell. If there is no room for a connection, the connection is handed over to another channel through directed retry already in the call setup phase. However, directed retry in the call setup phase must be made only on the basis of a few measuring reports. The handover target cell must hereby be chosen on a basis of very deficient measurements. On the other hand, at least some mobile stations having a connection with a base transceiver station of the cell would typically achieve a similar connection quality also with some other base transceiver station. From the viewpoint of the whole it is hereby most advantageous to move from the cell one or several mobile stations in an active state to adjacent cells and to set up the new connection without any handover in the call setup phase.
Another example of the advantages of handover due to excessive cell loading is a situation where there is one free channel both in cell S1 and in cell S2, and an attempt is made in cell S1 to set up a high-speed connection requiring the use of two channels. It is hereby possible with handover to move one of the connections in cell S1 to cell S2, whereby two free channels are obtained in cell S1 for use by the new connection.
The following is an examination of a state-of-the-art handover due to excessive cell loading as described in the GSM 08.08 (version 4.7.1) specification published by the European Telecommunications Standards Institute ETSI, the required signalling for which is shown in FIG. 3.
MSC inquires of base station controller BSC about the congestion situation of an individual cell by sending to the base station controller a RESOURCE REQUEST message 301, wherein the cell, the type of reporting and the reporting period, if any, are identified.
Base station controller BSC begins reporting on the utilisation rate of the cell""s capacity with the type of reporting stated by MSC. The base station controller is constantly monitoring the interference levels of free channels. Free channels are divided according to their interference level into five classes determined by the network management system NMS. BSC reports to MSC on the number of free channels in each interference level class in its transmitted RESOURCE INDICATION message 302.
The mobile services switching centre MSC studies the information which it has received on the load situation of the different cells and at point 303 in FIG. 3 it checks if the EXIT CRITERIA triggering handover due to cell loading are met for some cell. EXIT CRITERIA are a criterion that is defined in the mobile services switching centre MSC separately for each cell and that defines the situation where the cell load should be reduced. By establishing the criterion it is possible e.g. to determine the aim that at least 1 channel should always be kept free in the cell.
MSC appoints as target cell candidates a set {Si} of those cells wherein the ENTRY CRITERIA are met. ENTRY CRITERIA define a situation where the cell load may still be increased without problems with such handovers due to traffic which are allocated to the cell. Traffic-based handovers allocated to the cell can be allowed e.g. in such situations where there are more than 4 free channels in the cell in interference classes 1, 2 and 3. To be able to make a decision on handover due to cell loading, the mobile services switching centre MSC will thus also need information on the load situation of cells adjacent to the cell, besides the information it has received from the loaded cell itself.
Having found that handover due to loading is necessary, having concluded how many mobile stations must be moved from the cell and having found target cell candidates for handovers, the mobile services switching centre sends a request to the base station controller by handover to move a certain number of mobile stations out of the cell by sending a HANDOVER CANDIDATE ENQUIRY message 304 to the base station controller. In the message the mobile services switching centre names the cell from which the handovers are made, the number of mobile stations which should be moved out of the cell and the set {Si} of target cell candidates for the handover.
The base station controller at point 305 begins to move out of the congested cell the number of mobile stations requested by the mobile services switching centre MSC into the cells {Si} named in the HANDOVER CANDIDATE ENQUIRY message of the mobile services switching centre. The base station controller decides which mobile stations it will move from the congested cell and to which cells named by MSC it will move them. The base station controller bases its decision on those measuring reports of mobile stations which it knows.
A mobile station can be moved from cell S1 by handover into cell S2, if averaged measurements by the mobile station of signal strengths of cell S2 indicate the signal of cell S2 to be higher than the TRHO TARGET LEVEL(S2) limit value predetermined for it in cell S1. The TRHO TARGET LEVEL(Si) limit values are established for the adjacent cells Si of each cell, individually for each cell. The idea of using the TRHO TARGET LEVEL parameter is to make the handover due to cell loading into such a cell where the connection between mobile station and base transceiver station would be of too poor a quality.
Having decided which mobile stations it wishes to move out of the congested cell and which are the handover target cells for individual mobile stations, the base station controller begins the handovers. If a handover target cell for a mobile station is under the management of the same base station controller, BSC will send to mobile station MS through base transceiver station BTS a HANDOVER COMMAND message, wherein it states the new channel for use by the mobile station. If the target cell is subordinated to another base station controller BSC2, BSC will send to the mobile services switching centre a HANDOVER REQUIRED message with a xe2x80x9cresponse to MSC""s requestxe2x80x9d reason code, wherein it gives a list in an order of preference of possible handover target cells. Having begun the handovers, BSC states in the HANDOVER CANDIDATE RESPONSE message 306 to MSC how many mobile stations it is moving.
Implementation of handover due to traffic requires parametrisation both in the base station controller BSC and in the mobile services switching centre MSC. The TRHO TARGET LEVEL parameter must be defined in the base station controller BSC for each adjacent cell of the cells. If the TRHO TARGET LEVEL parameter is not defined for some adjacent cell Sj of cell Si, then no handover due to loading of cell Si can be done into cell Sj.
The EXIT CRITERIA condition must be defined in the mobile services switching centre for all those cells from which handover due to cell loading can be made. The ENTRY CRITERIA parameter must be defined for all those cells, into which load of other cells should be moved through handover due to cell loading. Since parametrisation involves much work and it must be done in several different places, handover due to cell loading is typically used in those cells only where there is a constant congestion. This is why state-of-the-art handover due to cell loading is hardly used at all for relieving short loading peaks caused by congestion situations e.g. due to traffic accidents.
Drawbacks of the method described above are the laborious nature of parametrisation required by the method and loading of interface A between the base station controller BSC and the mobile services switching centre MSC. Interface A will be unnecessarily loaded especially in handovers due to the internal cell loading of one base station controller BSC. Additional problems may be caused in networks where base station controllers and mobile services switching centres made by different manufacturers are used. Both the mobile services switching centre MSC and the base station controller BSC must hereby support handovers due to cell loading.
It is an objective of the present invention to eliminate or at least to alleviate the state-of-the-art problems mentioned above. This objective is attained with the method defined in the independent claim.
The inventive idea is to implement centrally in one base station controller BSC such a handover which is due to cell loading. The implementation requires implementation in the base station controller of all those functions and criteria which relate to handover due to cell loading.
Advantages achieved through centralisation are e.g. that the required signalling will be lighter. Since the method is independent of other network elements, all necessary parametrisation is done in one network element. This facilitates introduction and maintenance of the functionality.
As all information and functionality needed for deciding on handover are in the base station controller, the handover criteria may be defined as dynamic criteria. For example, when setting up new calls, a check is made in base station controller BSC to find out if the free capacity of the cell is sufficient for setting up a new connection, so this information may be used flexibly as a basis for the EXIT CRITERIA and ENTRY CRITERIA which will trigger off a handover due to cell loading. In this way call establishment is always possible in the best cell.